Most of them start immediately after you're done with the regular quests and class quests on a planet but the notable exceptions are Nar Shadda, this one starts after you finish Alderaan, you'll be sent back there on a class quest. Alderaan, this one is around level 37-40, you pick it up on your Fleet station. Finally Belsavis, this is the level 50 Bonus Series and will open up dailies.